In a 250 mL Erlenmeyer flask, I dissolved a small amount of corn starch in 150 mL of water. I then added a small amount of potassium iodide. The result was a slightly cloudy, but mostly clear solution.
In a 50 mL beaker, I made 40 mL of a saturated solution of sodium thiosulfate.
I then filled a medicine dropper with bleach.
The Erlenmeyer flask represented a person, and the bleach in the dropper represented sin. I added a few drops of bleach to the flask, which turned blue. The color change represented the effect of sin. As I added more drops, the color got deeper.
I then showed the kids the beaker, which represented Christ. He lived a sin-free life, which is why the solution was clear. I poured the clear solution into the colored solution that was in the flask, which represented Christ coming into a person’s life. When the two solutions mixed, the deep color went away, and the result was a solution that looked like what was originally in the Erlenmeyer flask – a slightly cloudy but mostly clear solution.
The chemistry involved in this demonstration is based on reduction and oxidation. Bleach is an oxidizing agent, which means it tends to take electrons away from other chemicals. When the iodide ion from the potassium iodide is oxidized, it becomes elemental iodine. A mixture of starch and iodine turns blue. The more iodine, the deeper the blue. Note that if the bleach is tinted, the color might not be blue. It might be something else, but that doesn’t matter. The demonstration will still work. So the bleach turns the iodide ion into iodine, which then turns the solution blue.
Sodium thiosulfate is a reducing agent. This means it tends to give electrons to other chemicals. When the solution of sodium thiosulfate is added to the mixture of iodine and starch, the sodium thiosulfate turns the iodine back into iodide. This returns the solution to its original state as far as the starch and iodide are concerned, which is why it returns to its original color.
The demonstration seemed to get the point across to the kids.